Tunable circuit for detection of negative voltages

ABSTRACT

The present invention provides a tunable circuit for quickly optimizing an electrical field generated by the F-N tunneling operation. To optimize this electrical field, the charging of a positive charge pump is begun after the charging of a negative charge pump. The tunable circuit of the present invention provides a means to detect the optimal negative voltage at which pumping of the positive voltage should begin. The tunable circuit includes a resistor chain coupled between a first reference voltage and a negative voltage from the negative charge pump. When charging of the negative charge pump begins, a comparator compares the voltage at a node within the resistor chain to a second reference voltage. In accordance with the present invention, the node voltage within the resistor chain is equal to the second reference voltage when the negative voltage is equal to the voltage to be detected. Thus, the comparator generates a trigger signal when the voltage at the node decreases to the second reference voltage. This output signal triggers the pumping of the positive charge pump. By changing the resistance within the resistor chain, the positive charge pumping may be initiated at varying negative voltages. In the present invention, additional resistance is added to or removed from the resistor chain via metal options or switches.

FIELD OF THE INVENTION

The present invention relates to an electrically erasable programmablefloating gate memory, such as flash memory or electrically erasableprogrammable read only memory (EEPROM) for both memory and programmablelogic application. More specifically, the present invention relates to amethod and structure to detect negative voltages to improve programmingof a memory cell.

DISCUSSION OF RELATED ART

Many memory cell arrays, such as flash memory cells, use Fowler-Nordheim(F-N) tunneling mechanisms to program and erase the memory cells. F-Ntunneling occurs when a large voltage differential exists between thecontrol gate terminal and a source/drain terminal of a memory cell. Thelarge voltage differential establishes an electrical field in the tunneloxide region of the memory cell. This electrical field promotes theacquisition of electrons by or distribution of electrons from thefloating gate of the memory cell, depending on the direction of thevoltage differential. When the voltage (e.g., +15 Volts) applied to thecontrol gate terminal is much larger than the voltage (e.g., 0 Volts)applied to the source/drain terminal, electrons are drawn into thefloating gate. When the voltage (e.g., 0 Volts) applied to the controlgate terminal is much smaller than the voltage (e.g., +15 Volts) appliedto the source/drain terminal, electrons are expelled from the floatinggate.

Some F-N tunneling schemes use positive and negative voltages to reducethe stress on chip elements. In these cases, the large voltagedifferential is created by having a positive voltage on one terminal ofthe memory cell and a negative voltage on another terminal. Thus, todraw electrons into the floating gate, a positive voltage (e.g., +5Volts) is applied to the control gate terminal of the memory cell and anegative voltage (e.g., −8 Volts) is applied to the source/drainterminal. Also, to remove electrons from the floating gate, a negativevoltage (e.g., −8 Volts) is applied to the control gate terminal of thememory cell and a positive voltage (e.g., +5 Volts) is applied to thesource/drain terminal.

Both a V_(CC) supply voltage (e.g., 3.3 Volts) and ground (0 Volts)voltages are readily available to any circuit on a chip. A charge pumpmust be used for voltages greater than the V_(CC) supply voltage or lessthan ground. Thus, charge pumps are needed to achieve the voltagedifferential required for an F-N tunneling operation.

A negative charge pump produces a negative supply voltage, V_(NN). TheV_(NN) negative supply voltage is the negative voltage used for the F-Ntunneling operation. The negative charge pump operates by graduallydecreasing (pumping down) the V_(NN) negative supply voltage from aninitial voltage of ground to the desired negative voltage.

A positive charge pump produces a positive supply voltage, V_(PP). TheV_(PP) positive supply voltage is the positive voltage used for the F-Ntunneling operation. The positive charge pump operates by graduallyincreasing (pumping up) the V_(PP) positive supply voltage from aninitial voltage of ground to the desired positive voltage. Therequirement of pumping these supply voltages causes a delay before thesesupply voltages reach their final desired voltages.

In a conventional F-N tunneling operation, the time between theinitiation of pumping of the negative charge pump and the initiation ofpumping between the positive charge pump may not be optimized. As aresult, a high negative voltage applied to the control gate and a highpositive voltage applied to the drain or source region to removeelectrons from the floating gate may cause an electrical field spike inthe tunnel oxide. This electrical field spike can induce “electrontrapping” in the tunnel oxide, wherein electrons from the F-N tunnelingoperation become trapped in the tunnel oxide adjacent to the top surfaceof the substrate of a memory cell. Electron trapping thus reduces theeffectiveness of the F-N tunneling operation by reducing the number ofelectrons that pass through the tunnel oxide of the memory cell duringthe F-N operation. Over many repetitions of the F-N operation,increasing numbers of electrons become trapped in the tunnel oxide ofthe memory cell. The amount of time a memory cell must undergo an F-Ntunneling operation increases with the number of electrons trapped inthe tunnel oxide of the memory cell. Each memory cell undergoing F-Ntunneling in an array is affected by electron trapping. As a result,electron trapping causes an entire memory cell to undergo increasinglylonger durations of F-N tunneling operations over time. Therefore, aneed arises for a way to minimize fluctuations in the electrical fieldduring programming of the memory cells.

SUMMARY OF THE INVENTION

The present invention provides a tunable circuit for optimizing anelectrical field generated by the F-N tunneling operation.

To optimize the electrical field, the charging of the positive chargepump is begun after the charging of the negative charge pump. Thetunable circuit of the present invention provides a means to detect theoptimal negative voltage at which pumping of the positive voltage shouldbegin. The tunable circuit includes a resistor chain coupled between afirst reference voltage and a negative voltage from a negative chargepump. A node within the resistor chain is compared to a second referencevoltage using a comparator. In this way, the node within the resistorchain is equal to the second reference voltage when the negative voltageis equal to the voltage to be detected.

The output signal of the comparator changes state when the voltage atthe node within the resistor chain decreases below the second referencevoltage. This output signal triggers the charging of a positive chargepump. Additional resistance is added to or removed from the resistorchain via metal options or switches. By changing the resistance withinthe resistor chain, the positive charge pumping may be initiated atvarying negative voltages.

Thus, the tunable circuit of the present invention provides a way totrip the charging of a positive charge pump based on a given voltagelevel of a negative charge pump during an F-N tunneling operation,thereby maintaining a constant electrical field during programming ofthe memory cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a negative voltage detector inaccordance with an embodiment of the present invention;

FIG. 2 is a plot of voltage over time for two charge pump voltages inaccordance with an embodiment of the present invention;

FIG. 3 is a plot of voltage over time for a family of charge pumpvoltages in accordance with an embodiment of the present invention;

FIG. 4 is a schematic diagram of a negative voltage detector inaccordance with another embodiment of the present invention; and

FIG. 5 is a schematic diagram of a negative voltage detector inaccordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Recent studies indicate that the electrical field formed by an F-Ntunneling operation has a more constant value throughout the operationif pumping of the positive voltage (or alternately the negative voltage)is initiated after the pumping of the negative voltage (or alternatelythe positive voltage) has begun. A more constant value of thiselectrical field has been shown to significantly reduce the incidence ofelectron trapping in memory cells undergoing the F-N tunnelingoperation. Additional, time-consuming testing is required to determinethe optimal negative voltage at which the pumping of the positivevoltage should begin. Specifically, to determine this optimal negativevoltage, many negative voltages are detected, and the constancy of eachassociated electrical field sampled.

The present invention provides a tunable negative voltage detector usedto efficiently detect the negative voltage at which charging of thepositive charge pump begins, thereby quickly optimizing the electricalfield. Determining the optimal negative voltage for the most constantelectrical field involves two steps. The first step is achieving someportion of the negative voltage required for the F-N tunneling operation(i.e., the trip voltage). The second step is initiating the pumping ofthe positive voltage required for the F-N tunneling operation when thenegative voltage passes through the trip voltage. During these twosteps, the constancy of the electrical field formed is measured. Thesetwo steps are repeated until the most constant electrical field isformed. The optimal negative voltage is then the negative voltageassociated with the most constant electrical field.

FIG. 1 illustrates a tunable negative voltage detector 100 in accordancewith the present invention coupled between a negative charge pump 104and a positive charge pump 105. Negative voltage detector 100 includes aresistor chain 101 and a comparator 102. Resistor chain 101 includesresistors R1-R30 coupled in series between a reference voltage generator103 and negative charge pump 104. In one embodiment, resistors R1-R30are p-well resistors with resistors R1-R3 having a resistance value of2500 Ohms, resistors R4-R5 and R14-R15 having a resistance value of10,000 Ohms, and resistors R6-R13 and R16-R30 having a resistance valueof 25,000 Ohms. Reference voltage generator 103, negative charge pump104, and positive charge pump 105 are well known in the art, and aretherefore not described in detail.

Reference voltage generator 103 provides a first reference voltage,V_(REF), to resistor chain 101. In one embodiment, reference voltagegenerator 103 is a band gap generator coupled with an amplifier andprovides a V_(REF) reference voltage of 2.1 Volts. Negative charge pump104 provides a negative supply voltage, V_(NN), to resistor chain 101.Negative charge pump 104 pumps down the V_(NN) negative supply voltagefrom an initial voltage (e.g., 0 Volts) to a final voltage (e.g., −8Volts). The value of the final voltage of the V_(NN) negative supplyvoltage is the negative voltage required by the F-N tunneling operation.Positive charge pump 105 pumps up the V_(PP) positive supply voltagefrom an initial voltage (e.g., 0 Volts) to a final voltage (e.g., 8Volts). The value of the final voltage of the V_(PP) positive supplyvoltage is the positive voltage required by the F-N tunneling operation.

The non-inverting terminal of comparator 102 is coupled to receive thevoltage present between resistors R11-R12. The inverting terminal ofcomparator 102 is coupled to ground. Comparator 102 is coupled toprovide the DETECT signal to positive charge pump 105.

Metal open options M01-M04 and metal short options MS1-MS11 selectivelytune negative voltage detector 100. A metal open option is a wire thatmay be cut to open a path. For example, in the circuit of FIG. 1, ifresistor R1 is shorted by metal open option M01, resistor R1 does notcontribute to the negative voltage detection. However, if metal openoption M01 is cut, then resistor R1 does contribute to the negativevoltage detection.

A metal short option is a junction that may be wired closed to short apath. For example, in the circuit of FIG. 1, resistor R3 contributes tothe negative voltage detection. However, if metal short option MS1 iswired closed, then resistor R3 is shorted and therefore does notcontribute to the detection of the negative voltage. Those skilled inthe art will recognize other embodiments of the present invention, suchas locating the metal open and metal short options in other locationsalong resistor chain 101.

Negative voltage detector 100 operates as follows. The resistance valueof each resistor R1-R30 and the V_(REF) reference voltage (e.g., 2.1Volts) are known. The V_(NN) negative supply voltage is initially equalto a voltage of 0 Volts. At this point, all nodes between resistorsR11-R12 must be positive because the nodes are located between theV_(REF) positive reference voltage and ground. Thus, the node betweenresistors R11-R12 is positive. As a result, the voltage provided to thenon-inverting terminal of comparator 102 is more positive than thevoltage of ground provided to the inverting terminal of comparator 102.Therefore, comparator 102 initially provides a logic low DETECT signalto positive charge pump 105. This logic low value of the DETECT signalprevents positive charge pump 105 from pumping.

As the voltage of the V_(NN) negative supply voltage is pumped down, thevoltage at each node of resistor chain 101 lessens. When the V_(NN)supply voltage reaches a given trip voltage, V_(TRIP), the voltage atthe node between resistors R11-R12 has decreased to a voltage of 0Volts. At this time (i.e., time T_(TRIP)) the DETECT signal provided bycomparator 102 transitions to a logic high value. This logic high valueof the DETECT signal triggers positive charge pump 105 to initiate thepumping up of the V_(PP) positive supply voltage.

Thus, the present invention causes the positive charge pump to pump upthe V_(PP) positive supply voltage only after the negative charge pumppumps down the V_(NN) negative supply voltage to a predetermined level.In this manner, the present invention significantly reduces electrontrapping during an F-N operation by keeping a constant electrical fieldthroughout the programming of the memory cells. As described above, thisconstant electrical field is achieved by pumping up the positive chargepump only after the negative charge pump pumps down the V_(NN) negativesupply voltage to a predetermined level.

FIG. 2 is a plot of voltage over time for two signals, the V_(NN)negative supply voltage and the V_(PP) positive supply voltage.Initially, negative charge pump 104 (FIG. 1) provides a V_(NN) negativesupply voltage equal to 0 Volts. The V_(NN) supply voltage graduallydecreases to a steady negative state (e.g., −8.0 Volts). In accordancewith the present invention, the charge pumping of the V_(PP) positivesupply voltage begins only after the V_(NN) negative supply voltage hasreached a certain trip voltage, V_(TRIP), as described above. The V_(NN)negative supply voltage reaches a voltage of V_(TRIP) at time T_(TRIP).Thus, at time T_(TRIP), positive charge pump 105 starts pumping up theV_(PP) positive supply voltage. The V_(PP) positive supply voltage hasan initial voltage of 0 Volts and gradually increases to a steadypositive state (e.g., +8.0 Volts).

FIG. 3 is a plot of voltage over time for a family of V_(PP) positivesupply voltage curves for three different V_(TRIP) trip voltages. TheV_(PP1)-V_(PP3) positive voltage supplies represent the V_(PP) positivesupply voltage as a function of the trip voltages V_(TRIP)-V_(TPRIP3).Specifically, V_(PP1) begins pumping up at time T_(TRIP1) when resistorchain 100 is set to detect the V_(TRIP1) trip voltage, V_(PP2) beginspumping up at time T_(TRIP2) when resistor chain 100 is set to detectthe V_(TRIP2) trip voltage, and V_(PP3) begins pumping up at timeT_(TRIP3) when resistor chain 100 is set to detect the V_(TRIP3) tripvoltage.

The present invention facilitates the quick analysis of any number oftrip voltages, thereby allowing the optimal trip voltage to bedetermined. Thus, the tunable nature of negative voltage detector 100allows the initiation of positive charge pumping of the V_(PP) positivesupply voltage at many different times. For example, assume resistorchain 100 as shown in FIG. 1 detects the V_(TRIP1) voltage at timeT_(TRIP1). If it is desired to initiate the pumping up of the V_(PP)positive supply voltage at an earlier time, T_(TRIP2), then the tripvoltage of the V_(NN) negative supply voltage detected by the presentinvention must be increased to a less negative voltage, V_(TRIP2). Todetect the V_(TRIP2) voltage with resistor chain 100 (FIG. 1),additional resistors are added into the resistor chain above the nodebetween resistors R11-R12. Metal open options M01-M02, which shortresistors R1-R2, respectively, are cut to provide the additionalresistance. As a result of cutting metal open options M01-M02, the tripvoltage increases to a less negative voltage, V_(TRIP2), therebyinitiating the pumping positive charge pump 105 at an earlier time,T_(TRIP2) The result is shown as the V_(PP2) positive supply voltage.Note that removing resistors below the node between resistors R11-R12also detects a less negative trip voltage than V_(TRIP1).

If it is desired to initiate the pumping up the V_(PP) positive supplyvoltage at a later time, T_(TRIP3), the trip voltage of the V_(NN)negative supply voltage detected by the present invention must bedecreased to a more negative voltage, V_(TRIP3). To detect the V_(TRIP3)voltage from resistor chain 100 (FIG. 1), resistors are removed from theresistor chain above the node between resistors R11-R12. Metal shortoptions MS7-MS9 are wired closed to short resistors R17-R19,respectively. As a result, the trip voltage of the V_(NN) negativesupply voltage detected by the present invention decreases to a morenegative voltage, V_(TRIP3). Therefore, the positive charge pump beginspumping up the V_(PP) positive supply voltage at a later time,T_(TRIP3). The result is shown as the V_(PP3) positive supply voltage.Note that removing resistors above the node between resistors R11-R12also detects a more negative voltage than V_(TRIP1).

The present invention advantageously detects different trip points ofthe V_(NN) negative supply voltage. As a result, the trip point whichoptimizes the electrical field during an F-N tunneling operation isquickly ascertained.

FIG. 4 illustrates a tunable negative voltage detector 400 in accordancewith another embodiment of the present invention. Similar elements inFIGS. 1 and 4 are labeled similarly. Thus, negative voltage detector 400is coupled between a negative charge pump 104 and a positive charge pump105. Negative voltage detector 400 includes a resistor chain 101 and acomparator 102.

Negative voltage detector 400 includes all of the functionalitydescribed with respect to negative voltage detector 100 (FIG. 1). Theaddition of metal open options M05-M09 and metal short option MS12beneficially provide an additional way to change the voltage detected bynegative voltage detector 400.

FIG. 5 illustrates a tunable negative voltage detector 500 in accordancewith another embodiment of the present invention. Similar elements inFIGS. 1 and 5 are labeled similarly. Thus, negative voltage detector 500is coupled between a negative charge pump 104 and a positive charge pump105. Negative voltage detector 500 includes a resistor chain 101 and acomparator 102.

Negative voltage detector 500 includes all of the functionalitydescribed with respect to negative voltage detector 100 (FIG. 1). Theaddition of metal switches S1-S15 beneficially provide an additional wayto change the voltage detected by negative voltage detector 500.

Although the present invention has been described in connection with oneembodiment, it is understood that this invention is not limited to suchembodiment, but is capable of various modifications which would beapparent to a person skilled in the art. For example, metal switches maybe used in place of metal open options and metal short options inresistor chain 100. Thus, the invention is limited only by the followingclaims.

We claim:
 1. A method of optimizing an electrical field formed by aFowler-Nordheim tunneling operation, the method comprising the steps of:generating the electrical field formed by the Fowler-Nordheim tunnelingoperation by pumping a negative voltage source to a first predeterminedvoltage level; and pumping a positive voltage source to a secondpredetermined voltage level after the first predetermined voltage levelhas been reached.
 2. The method of claim 1, wherein pumping the negativevoltage source to the first predetermined voltage level is performed bya negative charge pump.
 3. The method of claim 1, wherein pumping thepositive voltage source to the second predetermined voltage level isperformed by a positive charge pump.
 4. A tunable circuit for optimizingan electrical field formed by a Fowler-Nordheim tunneling operation,comprising: means to generate a negative voltage; a comparatorgenerating a trigger signal when the negative voltage decreases to avoltage at which pumping of a positive voltage should begin; and meansresponsive to the trigger signal to trigger pumping of a positive chargepump.
 5. A tunable circuit as in claim 4, further comprising a variableresistor coupled between a first reference voltage and the negativevoltage.
 6. The tunable circuit of claim 5, wherein the comparator takesa positive input from a node along the variable resistor.
 7. The tunablecircuit as in claim 5, wherein the variable resistor comprises a chainof resistors, some of which are in parallel with metal short options. 8.The tunable circuit as in claim 5, wherein the comparator generates thetrigger signal when the voltage at a node between the first referencevoltage and the negative voltage decreases to a second referencevoltage.
 9. The tunable circuit as in claim 4, wherein the means togenerate the negative voltage is a negative charge pump.